I need to redirect the old URL to the new URL. Will it do that automatically or do I need to set up an automatic redirect. Eg people who click on old links on other sites that other people have posted will they automatically be redirected to new URL?

If you're talking about forumotion's URL adress you will be able to acces both URL adresses: the old and the new one. Hovewer, if you will acces the old adress you will be automatically redirected to the new one.